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  <div class="section" id="cmp0053">
<span id="policy:CMP0053"></span><h1>CMP0053<a class="headerlink" href="#cmp0053" title="Permalink to this headline">¶</a></h1>
<p>Simplify variable reference and escape sequence evaluation.</p>
<p>CMake 3.1 introduced a much faster implementation of evaluation of the
<a class="reference internal" href="../manual/cmake-language.7.html#variable-references"><span class="std std-ref">Variable References</span></a> and <a class="reference internal" href="../manual/cmake-language.7.html#escape-sequences"><span class="std std-ref">Escape Sequences</span></a> documented in the
<span class="target" id="index-0-manual:cmake-language(7)"></span><a class="reference internal" href="../manual/cmake-language.7.html#manual:cmake-language(7)" title="cmake-language(7)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">cmake-language(7)</span></code></a> manual.  While the behavior is identical
to the legacy implementation in most cases, some corner cases were
cleaned up to simplify the behavior.  Specifically:</p>
<ul>
<li><p>Expansion of <code class="docutils literal notranslate"><span class="pre">&#64;VAR&#64;</span></code> reference syntax defined by the
<span class="target" id="index-0-command:configure_file"></span><a class="reference internal" href="../command/configure_file.html#command:configure_file" title="configure_file"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">configure_file()</span></code></a> and <span class="target" id="index-0-command:string"></span><a class="reference internal" href="../command/string.html#command:string" title="string"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">string(CONFIGURE)</span></code></a>
commands is no longer performed in other contexts.</p></li>
<li><p>Literal <code class="docutils literal notranslate"><span class="pre">${VAR}</span></code> reference syntax may contain only
alphanumeric characters (<code class="docutils literal notranslate"><span class="pre">A-Z</span></code>, <code class="docutils literal notranslate"><span class="pre">a-z</span></code>, <code class="docutils literal notranslate"><span class="pre">0-9</span></code>) and
the characters <code class="docutils literal notranslate"><span class="pre">_</span></code>, <code class="docutils literal notranslate"><span class="pre">.</span></code>, <code class="docutils literal notranslate"><span class="pre">/</span></code>, <code class="docutils literal notranslate"><span class="pre">-</span></code>, and <code class="docutils literal notranslate"><span class="pre">+</span></code>.
Note that <code class="docutils literal notranslate"><span class="pre">$</span></code> is technically allowed in the <code class="docutils literal notranslate"><span class="pre">NEW</span></code> behavior, but is
invalid for <code class="docutils literal notranslate"><span class="pre">OLD</span></code> behavior.  This is due to an oversight during the
implementation of <span class="target" id="index-0-policy:CMP0053"></span><a class="reference internal" href="#policy:CMP0053" title="CMP0053"><code class="xref cmake cmake-policy docutils literal notranslate"><span class="pre">CMP0053</span></code></a> and its use as a literal variable
reference is discouraged for this reason.
Variables with other characters in their name may still
be referenced indirectly, e.g.</p>
<div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">set(</span><span class="nb">varname</span><span class="w"> </span><span class="s">&quot;otherwise &amp; disallowed $ characters&quot;</span><span class="nf">)</span><span class="w"></span>
<span class="nf">message(</span><span class="s">&quot;${${varname}}&quot;</span><span class="nf">)</span><span class="w"></span>
</pre></div>
</div>
</li>
<li><p>The setting of policy <span class="target" id="index-0-policy:CMP0010"></span><a class="reference internal" href="CMP0010.html#policy:CMP0010" title="CMP0010"><code class="xref cmake cmake-policy docutils literal notranslate"><span class="pre">CMP0010</span></code></a> is not considered,
so improper variable reference syntax is always an error.</p></li>
<li><p>More characters are allowed to be escaped in variable names.
Previously, only <code class="docutils literal notranslate"><span class="pre">()#&quot;</span> <span class="pre">\&#64;^</span></code> were valid characters to
escape. Now any non-alphanumeric, non-semicolon, non-NUL
character may be escaped following the <code class="docutils literal notranslate"><span class="pre">escape_identity</span></code>
production in the <a class="reference internal" href="../manual/cmake-language.7.html#escape-sequences"><span class="std std-ref">Escape Sequences</span></a> section of the
<span class="target" id="index-1-manual:cmake-language(7)"></span><a class="reference internal" href="../manual/cmake-language.7.html#manual:cmake-language(7)" title="cmake-language(7)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">cmake-language(7)</span></code></a> manual.</p></li>
</ul>
<p>The <code class="docutils literal notranslate"><span class="pre">OLD</span></code> behavior for this policy is to honor the legacy behavior for
variable references and escape sequences.  The <code class="docutils literal notranslate"><span class="pre">NEW</span></code> behavior is to
use the simpler variable expansion and escape sequence evaluation rules.</p>
<p>This policy was introduced in CMake version 3.1.
CMake version 3.16.5 warns when the policy is not set and uses
<code class="docutils literal notranslate"><span class="pre">OLD</span></code> behavior.  Use the <span class="target" id="index-0-command:cmake_policy"></span><a class="reference internal" href="../command/cmake_policy.html#command:cmake_policy" title="cmake_policy"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">cmake_policy()</span></code></a> command to set
it to <code class="docutils literal notranslate"><span class="pre">OLD</span></code> or <code class="docutils literal notranslate"><span class="pre">NEW</span></code> explicitly.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The <code class="docutils literal notranslate"><span class="pre">OLD</span></code> behavior of a policy is
<span class="target" id="index-0-manual:cmake-policies(7)"></span><a class="reference internal" href="../manual/cmake-policies.7.html#manual:cmake-policies(7)" title="cmake-policies(7)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">deprecated</span> <span class="pre">by</span> <span class="pre">definition</span></code></a>
and may be removed in a future version of CMake.</p>
</div>
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